Tribological Mechanism of Graphene and Ionic Liquid Mixed Fluid on Grinding Interface under Nanofluid Minimum Quantity Lubrication

Abstract

Graphene has superhigh thermal conductivity up to 5000 W/(m·K), extremely thin thickness, superhigh mechanical strength and nano-lamellar structure with low interlayer shear strength, making it possess great potential in minimum quantity lubrication (MQL) grinding. Meanwhile, ionic liquids (ILs) have higher thermal conductivity and better thermal stability than vegetable oils, which are frequently used as MQL grinding fluids. And ILs have extremely low vapor pressure, thereby avoiding film boiling in grinding. These excellent properties make ILs also have immense potential in MQL grinding. However, the grinding performance of graphene and ionic liquid mixed fluid under nanofluid minimum quantity lubrication (NMQL), and its tribological mechanism on abrasive grain/workpiece grinding interface, are still unclear. This research firstly evaluates the grinding performance of graphene and ionic liquid mixed nanofluids (graphene/IL nanofluids) under NMQL experimentally. The evaluation shows that graphene/IL nanofluids can further strengthen both the cooling and lubricating performances compared with MQL grinding using ILs only. The specific grinding energy and grinding force ratio can be reduced by over 40% at grinding depth of 10 μm. Workpiece machined surface roughness can be decreased by over 10%, and grinding temperature can be lowered over 50 ℃ at grinding depth of 30 μm. Aiming at the unclear tribological mechanism of graphene/IL nanofluids, molecular dynamics simulations for abrasive grain/workpiece grinding interface are performed to explore the formation mechanism of physical adsorption film. The simulations show that the grinding interface is in a boundary lubrication state. IL molecules absorb in groove-like fractures on grain wear flat face to form boundary lubrication film, and graphene nanosheets can enter into the grinding interface to further decrease the contact area between abrasive grain and workpiece. Compared with MQL grinding, the average tangential grinding force of graphene/IL nanofluids can decrease up to 10.8%. The interlayer shear effect and low interlayer shear strength of graphene nanosheets are the principal causes of enhanced lubricating performance on the grinding interface. EDS and XPS analyses are further carried out to explore the formation mechanism of chemical reaction film. The analyses show that IL base fluid happens chemical reactions with workpiece material, producing FeF ₂, CrF ₃, and BN. The fresh machined surface of workpiece is oxidized by air, producing NiO, Cr ₂O ₃ and Fe ₂O ₃. The chemical reaction film is constituted by fluorides, nitrides and oxides together. The combined action of physical adsorption film and chemical reaction film make graphene/IL nanofluids obtain excellent grinding performance.